#pragma once

#ifndef T_THREAD_POOL_H
#define T_THREAD_POOL_H

#include <functional>
#include <iostream>
#include <thread>
#include <vector>
#include <queue>
#include <mutex>
#include <future>

class TThreadPool {
public:
    // the constructor just launches some amount of workers
    TThreadPool(size_t threads)
        :   stop(false)
    {
        for(size_t i = 0;i<threads;++i)
            workers.emplace_back(
                        [this]()
            {
                /* 一共有多个线程再跑，每个线程都是死循环，并从队列中取出函数来执行，从队列中取函数，如果取到了，条件变量往下执行，否则在等待 */
                for(;;)
                {
                    std::function<void()> task;

                    {
                        std::unique_lock<std::mutex> lock(this->queue_mutex);
                        this->condition.wait(lock,
                                             [this]{
                            return (this->stop || !this->tasks.empty());
                        });/* 当为false时，阻塞，当收到解除信号后，为true才会解除阻塞 */

                        if(this->stop && this->tasks.empty())
                            return;
                        task = std::move(this->tasks.front());
                        this->tasks.pop();
                    }

                    task();
                }
            }
            );
    }

    // add new work item to the pool
    template<class F, class... Args>
    std::future<typename std::result_of<F(Args...)>::type> Enqueue(F&& f, Args&&... args)
    {
        using return_type = typename std::result_of<F(Args...)>::type;
        auto task = std::make_shared< std::packaged_task<return_type()> >(
                    std::bind(std::forward<F>(f), std::forward<Args>(args)...)
                    );/* 创建一个智能指针类型,完美转发 */
        std::future<return_type> res = task->get_future(); /* 这里并没有去用res get，如果get将在接收端进行等待异步计算完成 */
        {
            std::unique_lock<std::mutex> lock(queue_mutex);

            // don't allow enqueueing after stopping the pool
            if(stop)
                throw std::runtime_error("enqueue on stopped ThreadPool");

            tasks.emplace([task](){ (*task)(); });
        }
        condition.notify_one();
        return res;
    }

    // the destructor joins all threads
    ~TThreadPool()
    {
        {
            std::unique_lock<std::mutex> lock(queue_mutex);
            stop = true;
        }
        condition.notify_all();
        for(std::thread &worker: workers)
            worker.join();
    };

private:
    // need to keep track of threads so we can join them
    std::vector< std::thread > workers;
    // the task queue
    std::queue< std::function<void()> > tasks;

    // synchronization
    std::mutex queue_mutex;
    std::condition_variable condition;
    bool stop;
};

#endif
